1. Field of the Invention
The present invention relates to ink sets for ink-jet recording, which are used in recording black images on a recording medium by using in combination plural inks different in polarity as black inks, and an ink-jet recording method and ink-jet recording instruments using such an ink set, and more particularly to ink sets for ink-jet recording, which permit the provision of clear and high-quality images having a sufficient optical density and of prints having excellent water fastness and light fastness, irrespective of the kind of plain paper, and an ink-jet recording method and ink-jet recording instruments using such an ink set.
The present invention also relates to ink sets for ink-jet recording in which a color image is printed with at least two inks of different colors by ejecting ink droplets on a recording medium, and an ink-jet recording method and ink-jet recording instruments using such an ink set, and more particularly to ink sets, which can prevent bleeding at boundaries between a black ink and color inks (hereinafter referred to xe2x80x9cbleedingxe2x80x9d simply) and white uneven image disorder (hereinafter referred to xe2x80x9cwhite hazexe2x80x9d) occurred in a black image region at a boundary area between a black ink and a color ink even when plain paper is used, and hence permits the provision of clear color images, and an ink-jet recording method and ink-jet recording instruments using such an ink set.
2. Related Background Art
With the spread of ink-jet recording systems, it is advanced to improve the quality and fastness properties of prints. With respect to, for example, water fastness, Japanese Patent Application Laid-Open No. 4-226175 and U.S. Pat. No. 5,053,495 disclose novel black dyes and inks which can provide images having good water fastness.
Black inks using a pigment have been reported to form black images excellent in print quality and fastness properties such as water fastness and light fastness on plain paper.
For example, Japanese Patent Application Laid-Open No. 5-179183 discloses pigment inks stabilized with a block polymer of an AB or BAB type, in which A is a hydrophobic homopolymer or copolymer of an acrylic monomer, and B is a hydrophilic polymer or a salt thereof. Japanese Patent Application Laid-Open No. 6-136311 discloses pigment inks which comprise a block polymer of a hydrophobic polymer having a basic amine functional group and a hydrophilic polymer having a nonionic acidic functional group, can provide prints of excellent quality and have excellent dispersion stability. Japanese Patent Application Laid-Open No. 7-53841 has proposed pigment inks for ink-jet, which comprise a triblock polymer of an ABC type, in which A is a hydrophilic homopolymer, B is a polymer bonded to solid particles, and C is a hydrophilic or hydrophobic polymer different from the block A, have excellent dispersion stability and can provide prints of excellent quality.
On the other hand, with the spread of ink-jet recording systems, it is attempted to improve the quality of color images formed on plain paper.
In order to improve the quality of color images, in particular, it is required that black characters recorded on plain paper have such good quality as they are high in density, sharp and free of any feathering. It has also been reported to reduce bleeding phenomenon caused by contact of plural recording inks of different colors with each other at boundary parts of different colors and white haze occurred in a black image region at a boundary area between a color ink and a black ink. White haze means a phenomenon that an image is disordered white unevenly.
For example, Japanese Patent Application Laid-Open No. 8-193176 discloses inks comprising a penetrating agent and 0.02 to 0.5% by weight of an alginate. Japanese Patent Application Laid-Open No. 6-57192 has proposed black inks containing an anionic dye and yellow inks containing a cationic dye and a polyvalent precipitant. Japanese Patent Application Laid-Open No. 9-118850 has proposed inks in which a first ink is composed of a pigment dispersion, and a second ink comprises an organic acid salt or mineral acid salt. Japanese Patent Application Laid-Open No. 7-314888 has proposed inks comprising a specific fluorine compound.
The methods described in Japanese Patent Application Laid-Open No. 4-226175 and U.S. Pat. No. 5,053,495 feature that one or more carboxylic acids, which are weak acids, are introduced into a dye molecule, thereby developing the water fastness of the resulting print making good use of the dependence of solubility of the dye on pH. However, the water fastness is insufficient, in particular, when using neutralized paper because the surface pH of paper plain varies with the kind of the paper.
The pigment inks disclosed in Japanese Patent Application Laid-Open Nos. 5-179183, 6-136311 and 7-53841 can provide images excellent in fastness properties such as water fastness and light fastness, and improve the print quality in plain paper to some extend. However, these inks have involved such problems as optical density becomes insufficient according to the kind of plain paper, and fixing ability becomes poor in plain paper that sufficient optical density can be attained.
On the other hand, the inks comprising a penetrating agent, 0.02 to 0.5% by weight of an alginate and a solid wetting agent described in Japanese Patent Application Laid-Open No. 8-193176 can prevent bleeding and white haze phenomenon to some extent. However, the wettability of paper is enhanced due to the penetrating agent and solid wetting agent, so that feathering occurs at edge parts of characters printed, resulting in a failure to attain satisfactory character quality.
The inks described in Japanese Patent Application Laid-Open Nos. 6-57192 and 9-118850 cause aggregation of dyes or pigments at boundaries between different colors, so that bleeding can be prevented to some extent, and the quality of black characters is also improved to some extent. However, the aggregation occurs only at boundaries within a black region, so that white haze phenomenon that occurs in the black region cannot be prevented. On the other hand, Japanese Patent Application Laid-Open No. 7-314888 discloses the inks comprising the specific fluorine compound, in which the surface energy of a first ink is adjusted within 15 dyn/cm of the surface energy of a second ink. White haze phenomenon is reduced to some extent by this method. However, it has been insufficient to prevent the white haze phenomenon in the case of high speed recording in which different colors overlap one after other in a short period of time, or according to the kind of plain paper.
It is therefore the first object of the present invention to provide an ink set for ink-jet, which can attain high optical density for images of black ink and has good fixing ability, irrespective of the kind of plain paper, and a recording method and recording instruments using such an ink set.
The second object of the present invention is to provide an ink set for ink-jet, which can attain high optical density for images of black ink, has good fixing ability and can provide prints excellent in fastness properties such as water fastness and light fastness, irrespective of the kind of plain paper, and a recording method and recording instruments using such an ink set.
The third object of the present invention is to provide an ink set which can provide black characters of high quality, prevent both bleeding and white haze and provide clear color images, irrespective of the kind of plain paper, and a recording method and recording instruments using such an ink set.
The above objects can be achieved by the present invention described below.
According to the first aspect of the present invention, there is provided an ink set comprising two black inks different in polarity from each other as black inks.
The ink set according to the first aspect of the present invention includes the following three embodiments.
(1) An ink set comprising two black inks different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is also a pigment ink.
(2) An ink set comprising two black inks different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is a dye ink.
(3) An ink set comprising two black inks different in polarity according to the embodiment (1) or (2), and additional color inks.
The ink set according to the embodiment (1) will be first described.
The ink set according to the embodiment (1) comprises two black inks different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is also a pigment ink.
The ink set may comprise two black inks the coloring materials for which are different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is also a pigment ink.
More specifically, the ink set may comprise two black inks the coloring materials for which are both pigments, wherein one black ink is cationic, and the other black ink is anionic.
The cationic ink may preferably be a pigment ink in which a pigment is dispersed with a cationic polymer or surfactant, or a self-dispersing type pigment ink in which a hydrophilic group is bonded to the surface of a pigment through an atomic group having a cationic group.
The anionic ink may preferably be a pigment ink in which a pigment is dispersed with an anionic polymer or surfactant, or a self-dispersing type pigment ink in which a hydrophilic group is bonded to the surface of a pigment through an atomic group having an anionic group.
The ink set may preferably comprise two black inks the coloring materials for which are both pigments, wherein one black ink is a self-dispersing type cationic pigment ink in which a hydrophilic group having a cationic group is directly bonded to carbon black, and the other black ink is a self-dispersing type anionic pigment ink in which a hydrophilic group having a anionic group is directly bonded to carbon black.
The hydrophilic group in the cationic pigment ink may preferably be composed of at least one aromatic or heterocyclic group and at least one cationic group.
The cationic group may preferably be a quaternary ammonium group, and the anionic group may be a sulfonic or carboxyl group.
One black ink of the two black inks in the ink set may preferably have a surface tension of at least 40 dyn/cm, and the other black ink may have a surface tension lower than 40 dyn/cm.
The ink set according to the embodiment (2) will be then described.
The ink set according to the embodiment (2) comprises two black inks different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is a dye ink.
The ink set may comprise two black inks the coloring materials for which are different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is a dye ink.
More specifically, the ink set may comprise two black inks the coloring materials for which are a pigment for one ink and a dye for the other ink, wherein one black ink is cationic, and the other black ink is anionic.
The cationic ink may preferably be a pigment ink in which a pigment is dispersed with a cationic polymer or surfactant, or a self-dispersing type pigment ink in which a hydrophilic group is bonded to the surface of a pigment through an atomic group having a cationic group.
The cationic ink may preferably be a cationic dye ink.
The anionic ink may preferably be a pigment ink in which a pigment is dispersed with an anionic polymer or surfactant, or a self-dispersing type pigment ink in which a hydrophilic group is bonded to the surface of a pigment through an atomic group having an anionic group.
The anionic ink may preferably be an anionic dye ink.
The ink set may preferably comprise two black inks the coloring materials for which are a pigment for one ink and a dye for the other ink, wherein one black ink is a self-dispersing type cationic pigment ink in which a hydrophilic group having a cationic group is directly bonded to carbon black, and the other black ink is an anionic dye ink.
The hydrophilic group in the cationic pigment ink may preferably be composed of at least one aromatic or heterocyclic group and at least one cationic group.
The cationic group may preferably be a quaternary ammonium group.
The cationic ink may preferably comprise a cationic polymer.
The anionic dye ink may preferably comprise an anionic polymer.
The ink set may preferably comprise two black inks the coloring materials for which are a pigment for one ink and a dye for the other ink, wherein one black ink is a cationic dye ink, and the other black ink is a self-dispersing type anionic pigment ink in which a hydrophilic group having an anionic group is directly bonded to carbon black.
The anionic group may preferably be a sulfonic or carboxylic group.
The anionic pigment ink may preferably comprise an anionic polymer.
The cationic dye ink may preferably comprise a cationic polymer.
One black ink of the two black inks in the ink set may preferably have a surface tension of at least 40 dyn/cm, and the other black ink may have a surface tension lower than 40 dyn/cm.
The ink set according to the embodiment (3) will be described.
The ink set according to the embodiment (3) comprises two black inks different in polarity from each other as black inks and additional color inks.
One of said two black inks different in polarity from each other may preferably be a pigment ink, and the other may also be a pigment ink. One of said two black inks the coloring materials for which are different in polarity may preferably be a pigment ink, and the other may also be a pigment ink. In this case, specific embodiments in which said two black inks different in polarity each other are both pigment inks may be the same as embodiments in the ink set according to the embodiment (1).
One of said two black inks different in polarity from each other may preferably be a pigment ink, and the other may be a dye ink. One of said two black inks the coloring materials for which are different in polarity may preferably be a pigment ink, and the other may be a dye ink. In this case, specific embodiments in which one of said two black inks different in polarity is a pigment ink, and the other is a dye ink may be the same as embodiments in the ink set according to the embodiment (2).
The coloring materials used for the color inks may preferably be dyes.
The color inks may preferably be at least yellow, magenta and cyan inks.
The color inks may preferably have a surface tension lower than 40 dyn/cm.
The present invention also relates to an ink-jet recording method and instruments using any one of these ink sets.
According to the second aspect of the present invention, there is thus provided an ink-jet recording method comprising ejecting ink droplets from orifices in response to recording signals to conduct recording on a recording medium, wherein two black inks different in polarity from each other are used as black inks to conduct the recording.
There is also provided an ink-jet recording method comprising ejecting ink droplets from orifices in response to recording signals to conduct recording of a black color on a recording medium, wherein two black inks different in polarity from each other are used as black inks to conduct the black-color recording.
The black inks may preferably be the ink set according to the embodiment (1) or (2).
There is further provided an ink-jet recording method comprising ejecting ink droplets from orifices in response to recording signals to conduct recording of at least two colors on a recording medium, wherein two inks different in polarity from each other as black inks, and color inks are used to conduct the recording.
The black inks and color inks may preferably be the ink set according to the embodiment (3).
The ink droplets may preferably be ejected by applying thermal energy to the inks.
According to the third aspect of the present invention, there is provided a recording unit comprising an ink container portion with an ink held therein and a head from which the ink is ejected in the form of ink droplets, wherein the ink is one of any ink set described above.
The head may preferably be a head of the type that thermal energy is applied to an ink to eject droplets of the ink.
The ink container portion may preferably be formed of polyurethane, cellulose, polyvinyl acetate or polyolefin resin.
According to the fourth aspect of the present invention, there is provided an ink cartridge comprising an ink container portion with an ink held therein, wherein said ink is one of any ink set described above.
The ink container portion may preferably be formed with a polyolefin at its surface with which the ink comes into contact.
According to the fifth aspect of the present invention, there is provided an ink-jet recording apparatus comprising a recording unit having an ink container portion with an ink held therein and a head from which the ink is ejected in the form of ink droplets, wherein the ink is one of any ink set described above.
The head may preferably be a head of the type that thermal energy is applied to an ink to eject droplets of the ink.
The ink container portion may preferably be formed of polyurethane, cellulose, polyvinyl acetate or polyolefin resin.
According to the sixth aspect of the present invention, there is provided an ink-jet recording apparatus comprising a recording head from which an ink is ejected in the form of ink droplets, an ink cartridge having an ink container portion with the ink held therein, and an ink feeder for feeding the ink contained in the ink cartridge to the recording head, wherein the ink is one of any ink set described above.
The recording head may preferably be a head of the type that thermal energy is applied to an ink to eject droplets of the ink.
The ink container portion may preferably be formed with a polyolefin at its surface with which the ink comes into contact.
With respect to the embodiment (1), the present inventors have carried out an extensive investigation with a view toward developing black inks which can attain high optical density for black images and provide high-quality images, irrespective of the kind of plain paper. As a result, it has been found that when a set of two black inks different in a polarity each other is used as a black ink, the performance satisfying the above object can be achieved.
In the present invention, the term xe2x80x9cpolarityxe2x80x9d means an ionicity that an ink has, and the term xe2x80x9cdifferent in a polarityxe2x80x9d means that at least two inks are different in an ionicity. For example, a combination of an anionic ink and a cationic ink may be mentioned.
More specifically, two black inks of an anionic black ink and a cationic black ink are used as black inks to form an image of a black color. As a method for changing the ionicity of an ink, may be mentioned a method in which the ionicity is changed with a coloring material, additive and/or the like. It is preferable that the ionicity of the coloring material be changed. It is more preferable that a pigment be used as the coloring material.
When recording of a black color is conducted by using both anionic black ink and cationic black ink as black inks, both inks come into contact with each other on a recording medium, and so the coloring materials aggregate in themselves due to bonding between anion and cation. As a result, the coloring materials remain on the surface of the recording medium, and only liquid media penetrate into the recording medium, so that a high optical density can be attained. When pigments are used as the coloring materials, the effect is more enhanced, and the fastness properties of the resulting print are improved. Since the aggregation of the coloring materials themselves occurs independent of the sizing agent and surface pH of plain paper, good optical density and image quality can be attained, irrespective of the kind of plain paper. When the penetrability of one of the two black inks is made higher, the fixing ability is also improved.
With respect to the embodiment (2), the present inventors have carried out an extensive investigation with a view toward developing black inks which can attain high optical density for black images and provide high-quality images excellent in fastness properties such as water fastness and light fastness, irrespective of the kind of plain paper. As a result, it has been found that when an ink set comprising two black inks different in polarity from each other, wherein one of said two black inks is a pigment ink, and the other is a dye ink, is used as a black ink, the performance satisfying the above object can be achieved. More specifically, an ink set composed of two black inks the coloring materials of which are a pigment for one ink and a dye for the other ink, wherein one black ink is anionic, and the other black ink is cationic, is used to form an image of a black color. As a method for changing the ionicity of an ink, may be mentioned a method in which the ionicity is changed with a coloring material, additive and/or the like. It is preferable that the ionicity of the coloring material be changed.
When recording of a black color is conducted by using both anionic black ink and cationic black ink as black inks, both inks come into contact with each other on a recording medium, and so the coloring materials aggregate in themselves. As a result, the coloring materials remain on the surface of the recording medium, and only liquid media penetrate into the recording medium, so that a high optical density can be attained. When a pigment is used as the coloring material for one ink, and a dye is uses as the coloring material for the other ink, the effect is more enhanced, and the fastness properties of the resulting print are also improved. Since the aggregation of the coloring materials themselves occurs independent of the sizing agent and surface pH of plain paper, good optical density and image quality can be attained, irrespective of the kind of plain paper.
When the penetrability of one of the two black inks is made higher, the fixing ability is also improved.
With respect to the embodiment (3), the present inventors have carried out an extensive investigation with a view toward attaining the excellent quality of black characters, irrespective of the kind of plain paper, and preventing bleeding and white haze phenomenon between a black ink and a color ink. As a result, it has been found that when two black inks different in polarity from each other are used as black inks in an ink set for ink-jet used in conducting recording of color images, the performance satisfying the above object can be achieved.
More specifically, two black inks of an anionic black ink and a cationic black ink are used as black inks to form an image of a black color. As a method for changing the ionicity of an ink, may be mentioned a method in which the ionicity is changed with a coloring material, additive and/or the like. It is preferable that the ionicity of the coloring material be changed. It is more preferable that a pigment be used as the coloring material.
When color recording of at least two colors is conducted by using both anionic black ink and cationic black ink as black inks, the black inks come into contact with each other on a recording medium, and so the coloring materials aggregate in themselves due to bonding between anion and cation and are fixed on the surface of the recording medium. As a result, only liquid media penetrate into the recording medium, so that high-quality black characters can be provided. Since the coloring materials of the black inks are fixed in all the region at boundary areas between the black ink and the color inks, both bleeding and white haze phenomenon can be substantially perfectly prevented. Since this effect is independent of the kind (for example, sizing agent and surface pH) of plain paper, the same effect can be brought about even in any plain paper. When the surface tensions of the color inks are adjusted to lower than 40 dyn/cm, the effect is more enhanced. When the penetrability of one of the two black inks is made higher, the fixing ability is also improved.